Quote:
Originally Posted by cgmg21
I am not a mechanic...
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You don't have to be a mechanic -- just read up on octane. Octane numbers indicate how likely it is to autoignite -- higher numbers are harder to ignite than lower numbers. Heat (from the air fuel mixture itself, or from lingering flame kernels from spark events) and cylinder pressure (either from piston characteristics or boost) can both cause fuel to auto ignite -- we only want the fuel to ignite when a timed spark event (i.e., when a spark plug fires) happens, not before and not after.
More ignition advance means less time for heat to dissipate from the piston head; higher CR's and or boost mean greater cylinder pressures. Therefore, octane requirements will depend on the CR of pistons, boost (if any) and ignition timing (other factors, such as atmospheric elevation, overall engine temps, etc. matter quite a bit too, but these are the key issues to keep in mind).
If the fuel ignites before or after the timed spark event, the pistons move out of time, creating stress on the engine internals, particularly the piston and rods. That phenomenon causes the "knock" sound you hear and it can result in cracked pistons or ruined rod bearings. Really bad autoignition events can create intense flame kernels that literally melt the piston head -- it will look like someone took an arc welder to it.
Anyway, the moral of the story is, you should use an octane no lower than recommended for the motor based on how it has been built and tuned. The requirements were determined empirically on an engine dyno when the motor was built and tested, and if you add forced induction, you pretty much have to once again determine octane requirements by observing power output and evidence of knock to determine new octane requirements to run safely.
Just saying "Ah, it'll be fine" is really taking an unnecessary risk.
It's worth reading up on so you know what you are doing and why it matters, and if you like cars, it's interesting stuff
